Training a character at the Boot Camp lets them level up farther. All the basic characters can level up maximum to 5. Training them up at the Boot Camp for the first time lets them reach 10, with that obvious cost issue. I agree on it being pretty expensive, at least for the level 15 which is about 20k. Remember to sell all the Valuables you find or the weapons you're not gonna need, you'll see how easy it is to get the money.

Also actually recruiting new characters which have the level cap unlocked is usually cheaper than training. (those which can level up to 10 have their name in green, those who can go up to 15 have it in blue)

Character training is suppose to be expensive, since at the later ingame recruitment officers (in different towns)you can get characters that can reach the higher ranks(levels) which you are meant to recruit some of instead of training those you have.The bootcamps were not a part of the game originally but something that was implemented due to people wanting to keep their 4 starting characters all the way through the game(which I personally don't understand why anyone would as it restricts you to playing the game in just 1 way).

Newly recruited characters do not autolevel to your current 3 characters level when you buy them, but considering it takes < 20 mins to level them up to the level you currently have, not really a big deal. Honestly I love the fact that there is a high cost involved with training, as it makes you look in other directions (like recruiting new people) and maybe try out different combinations of your crew. One of the games strengths is that you dont necessarily have to play bruiser/medikus/regulator(or bruiser/medikus/slayer) but that it actually can be played as regulator/regulator/regulator or bruiser/bruiser/bruiser if that is what you fancy.